Method reference double-colon syntax

[Brian Goetz]

> Another way to use the # in a future release would be to introduce literals for members of classes.

Well-covered ground.  Through target typing, it would indeed be possible 
to convert method references (Foo::bar) into Method objects or 
MethodHandle objects.  (The same syntax can be extended to fields being 
converted to Field objects.)  We've chosen to not do so for 8, but were 
careful to leave room for this in the future.